A piezoelectric device converts electric energy into mechanical energy such as mechanical displacement, stress, or vibration, or converts mechanical energy into electric energy, and is applied to an ultrasonic motor, an ink jet head, or the like.
Hitherto, a piezoelectric material containing lead as a main component, typified by lead zirconate titanate (Pb(TixZr1-x)O3), has been widely used for the piezoelectric device. In contrast, it has been reported that piezoelectric characteristics were enhanced by including an alkaline metal in a piezoelectric material, as a representative of a lead-free piezoelectric material (Japanese Patent No. 4,135,389). However, the inclusion of an alkaline metal causes a problem in its practical use, because there is a limit to use environment thereof due to hygroscopicity and the like. Furthermore, in a tungsten bronze structure metal oxide, an attempt has been made to control the orientation for the purpose of enhancing its function (Japanese Patent Application Laid-Open No. 2006-264316).
A conventional piezoelectric material mainly contains lead, and it is expected that the piezoelectric material contain no lead from the viewpoint of environmental load. Meanwhile, it has been reported that piezoelectric characteristics were enhanced with an alkaline metal-containing oxide piezoelectric material containing no lead. However, in a piezoelectric material containing an alkaline metal, compounds of Na and K that are raw materials are water-soluble. Therefore, there is a problem in terms of industrial production that simple steps of mixing raw materials in water, and thereafter, drying a mixed slurry cannot be adopted. Furthermore, there is a fear in that Na ions and K ions in a crystal are segregated at the grain boundary and the interface with electrodes due to migration during the use for a long period of time, and the piezoelectric material may absorb moisture. Thus, when a device is produced from such material, it is considered to be difficult to put the device into a practical use because the device exhibits poor stability and durability.
Furthermore, a lead-fee, alkali-free tungsten bronze structure metal oxide has such a feature that the shape anisotropy is large and a polarization axis direction is a C-axis direction that is a short direction of a unit cell of a crystal. Therefore, it is considered that a lead-fee tungsten bronze structure metal oxide can only have a 180° domain. Therefore, in a lead-fee, alkali-free tungsten bronze structure metal oxide having a random orientation formed by an ordinary production method for ceramics, the number of effective domains which may contribute to piezoelectric characteristics is small, and a large piezoelectric constant cannot be obtained.